1. Field of the Invention
This invention relates to an gradient index lens suitable as the collimator lens of a semiconductor laser or the pickup objective lens of an optical disc.
2. Description of the Prior Art
SELFOC lens (trade name) is well known as a lens having an index gradient in a direction perpendicular to the optic axis thereof, i.e., a so-called radial gradient index lens, and is used as an erect one-to-one magnification imaging element in a copying apparatus or the like. A merit of such a gradient index lens is that it is a single lens whose opposite end surfaces are planar surfaces and yet it has a relatively good performance and is excellent in mass productivity.
Attempts have also been made to make the best use of such a merit and use the gradient index lens as the collimator lens of a semiconductor laser or the pickup objective lens of an optical disc.
It is desired that the lens used for such a usage have a long working distance. If an attempt is made to form a lens having a long working distance by an gradient index single lens whose opposite surfaces are planar surfaces, the length d in the direction of the optic axis must be shortened. However, if the length d is shortened, the index gradient in a direction orthogonal to the optic axis will become steep and not only manufacture will become difficult but also aberrations cannot be well corrected.
Also, in the application of a lens of this type, it is necessary in terms of performance that particularly spherical aberration and sine condition be well corrected, but where a radial gradient single lens is used, spherical aberration and sine condition cannot be corrected at a time if the opposite surfaces of the single lens are planar surfaces.